This invention relates generally to probes for measuring underwater sounds, and more particularly, to a neutrally buoyant probe for directly measuring true underwater acoustic intensity.
Conventional underwater probes are suspended in a body of water, or are allowed to rest upon the floor of a body of water. The measurements that these probes provide can be used to determine sources of noise emission from underwater structures. For instance, the probes can be used to detect distant vessels, perform seismic surveys, or aid in fossil fuel exploration. The probes can also be used to assess the effectiveness and directionality of radiation from underwater sources.
An acoustic intensity measurement provides an indication of the vector energy flux density of an acoustic field. Acoustic intensity is the product of acoustic velocity and acoustic pressure.
The most commonly used technique known in the art for measuring underwater acoustic intensity is the two-hydrophone technique. This technique utilizes a probe consisting of two hydrophones fixed a distance apart. The sum of the outputs provides a direct measurement of the acoustic pressure at the midpoint of the hydrophones. Although the acoustic velocity is not directly measured using this technique, it can be derived. The acoustic velocity can be determined as: ##EQU1## where (v) is the acoustic velocity; (P) is the difference between the hydrophone outputs; (.omega.) is 2.pi. times the frequency; (.rho.) is the density of water; and (1) is the distance the hydrophones are set apart. The acoustic intensity is then determined as the time-averaged product of the acoustic pressure and velocity.
The two-hydrophone technique has several inherent disadvantages. The derived velocity is a function of frequency, and therefore, each spectral component to be studied must be weighted differently to compensate for the frequency dependence. The distance the hydrophones are set apart can cause inaccurate measurements. Setting the hydrophones too closely together can result in greatly accentuated phase and amplitude errors in the acoustic pressure signal. Setting the hydrophones too far apart can result in the inaccurate measurement of acoustic fields which depart from perfect plane wave behavior.
Another probe for measuring underwater acoustic intensity is disclosed in U.S. Pat. No. 3,311,873. This probe directly measures acoustic pressure and acoustic acceleration. The probe has additional signal processing for integrating acceleration to determine velocity. The acoustic intensity can then be determined from the measured pressure and calculated velocity. This type of probe often introduces phase errors into the intensity calculation, especially in highly reactive fields where pressure and velocity are nearly 90 degrees out of phase. Also, the sensitivity of this probe is frequency dependent, and provides less accurate measurements with lower frequency acoustic waves.
A neutrally buoyant probe is a probe which has an effective density substantially equal to the density of water. A probe which is neutrally buoyant responds to wave motion just as the body of water displaced by the probe would have responded.
An underwater acoustic velocity probe having a neutrally buoyant body is known in the art, and has been described, for example, in U.S. Pat. No. 2,582,994. The probe of U.S. Pat. No. 2,582,994 is an air-tight, rigid, metal sphere encapsulating an acoustic velocity sensor. The disadvantage of such a probe is that a rigid sphere perturbs the acoustic field to a significant degree, causing scattering of wave motion. Further disadvantages of the neutrally buoyant, rigid, spherical probe are that it is large, heavy out of the water, and difficult to machine.
The principal object of this invention is to provide an improved underwater probe for measuring true acoustic intensity. Another object is to provide for simpler and more accurate acoustic intensity computation by directly measuring both acoustic pressure and acoustic velocity. Still another object of the invention is to provide a probe which accurately measures acoustic intensity in highly reactive acoustic fields and over a broad band of frequencies. A further object is to provide a probe which is neutrally buoyant, readily manufactured, compact, and easily handled out of water.